Merge branch 'jn/eject-fetch-write-commit-graph-out-of-experimental' into master

"fetch.writeCommitGraph" was enabled when "feature.experimental" is asked for, but it was found to be a bit too risky even for bold folks in its current shape. The configuration has been ejected, at least for now, from the "experimental" feature set. * jn/eject-fetch-write-commit-graph-out-of-experimental: experimental: default to fetch.writeCommitGraph=false